This invention relates to an exposure apparatus and a device manufacturing method. More particularly, the invention is concerned with an exposure apparatus for use in an exposure process for the manufacture of various devices such as semiconductor devices (e.g., ICs or LSIs), image pickup devices (e.g., CCDs), display devices (e.g., liquid crystal panels), or sensors (e.g., magnetic heads), for example, and it is concerned with a device manufacturing method for manufacturing devices such as described above.
For further increases of integration and operation speed of a solid device such as an LSI, for example, miniaturization of a circuit pattern is being advanced. Currently, pattern formation uses a lithographic process which is based on a reduction projection exposure method having superiority in productivity and resolution performance. In this method, a circuit pattern of a mask is simultaneously transferred to a workpiece such as a semiconductor wafer by use of a projection lens. The limit resolution performance is proportional to the wavelength of exposure light and is in an inverse proportion to the numerical aperture (NA) of the projection lens. In consideration of this, improvement of resolution has been attempted by enlargement of the numerical aperture of the projection lens. However, further miniaturization of semiconductor devices needs shortening of the wavelength of the exposure light.
Recently, an addition to illumination light of a Hg lamp of e-line (wavelength xcex=546 nm), g-line (xcex=435 nm), h-line (xcex=405 nm) or i-line (xcex=365 nm), short wavelength excimer lasers (e.g., a KrF excimer laser with xcexxe2x88x92248 nm) are used practically in an exposure apparatus.
Even if a high resolution projection lens and a short wavelength and high luminance light source are used, uniform resolution is not attainable over the whole circuit pattern image as printed on a wafer unless the illuminance distribution (light intensity distribution) of light illuminating the photomask is not uniform. Thus, good pattern printing is not accomplished. For this reason, an illumination optical system of a projection exposure apparatus must provide illuminance uniformness of illumination light to be projected to a mask surface, to assure uniform exposure over the whole printing surface. To this end, conventionally an illumination optical system is disposed between a light source and a mask surface, which optical system comprises a condenser lens and an optical integrator including an optical fiber bundle or a lens array called a fly""s eye lens. In excimer laser exposure apparatuses, as a result of the use of a larger numerical aperture of a projection lens and a shorter wavelength of the light source for higher resolution precision and productivity, it is much more important to maintain illuminance uniformness (not greater than xc2x11%) than in other exposure apparatuses.
However, practically it is very difficult to maintain illuminance uniformness of not greater than xc2x11% for a long time period.
Short wavelength exposure light such as described above may be extracted out of a wide bandwidth range of a light source, comprising a Hg lamp, for example, by use of a filter for transmitting only a desired wavelength or a wavelength selective thin film (optical thin film deposited on a lens surface or a mirror surface for transmitting or reflecting a desired wavelength only). Exposure light emitted from a light source goes through an illumination optical system for illuminating a reticle and a projection optical system (projection lens) for imaging a fine pattern, as formed on the reticle, upon a photosensitive substrate, by which transfer and printing of the fine pattern onto the photosensitive substrate is performed. In exposure apparatuses, because of miniaturization of the pattern line width, improvements of throughput and resolution are required, and this necessitates the use of higher power exposure light and, additionally, a narrowed bandwidth of exposure light.
It is known that, in an exposure apparatus using exposure light of i-line (wavelength xcex=365 nm) and an exposure apparatus using exposure light of a wavelength shorter than the i-line, as a result of the shortening of the wavelength, the exposure light causes an opto-chemical reaction between oxygen and impurities in the air. There is an inconvenience that the product (blurring materials) created by this reaction adheres to a glass material to cause an opaque xe2x80x9cblurxe2x80x9d on the glass material. As for such a blurring material, there by ammonium sulfate (NH4)2SO4, as a representative, which can be produced when, for example, sulfurous acid absorbs energy of the light and turns into an excited state such that it reacts with oxygen in the air (i.e., is oxidized). Such ammonium sulfate bears a white color, and it produces a xe2x80x9cblurxe2x80x9d when deposited on the surface of an optical component such as a lens or a mirror. In that case, the exposure light is scattered or absorbed by the ammonium sulfate and, as a result, the transmission factor of the optical system decreases.
Particularly, in the short wavelength region as of a KrF excimer laser, for example, wherein the exposure light has a wavelength of about 248 nm which is shorter than the i-line, the exposure light may cause a strong opto-chemical reaction. In addition to production of xe2x80x9cblurxe2x80x9d as described above, there may be a phenomenon that the exposure light causes a further reaction of oxygen in the air to produce ozone such that both residual oxygen and produced ozone absorb the exposure light. This results in a further decrease of the light quantity (transmission factor) of exposure light which can reach the photosensitive substrate, and it causes a reduction of throughput. Japanese Laid-Open Patent Application, Laid-Open No. 216000/1994 proposes the use of a casing of a sealingly closed structure which accommodates therein a barrel containing a glass member such as a lens, for example, inside of the casing being filled with an inert gas to keep a low oxygen density to thereby prevent ozone generation.
With this method, however, attenuation or variation of exposure light resulting from absorption of exposure light by xe2x80x9cblurxe2x80x9d or ozone cannot be prevented sufficiently.
It is accordingly an object of the present invention to provide an exposure apparatus and/or a device manufacturing method by which any change in illuminance or illuminance non-uniformness can be suppressed such that a substrate can be exposed constantly with a correct exposure amount.
In accordance with an aspect of the present invention, there is provided an exposure apparatus, comprising: an illumination optical system for illuminating an original with ultraviolet light; a projection optical system for projecting a pattern of the original onto a substrate to be exposed; and gas purging means for replacing an inside space, where optical components of at least one of said illumination optical system and said projection optical system are placed, with a gas containing substantially no moisture or water content.
In accordance with another aspect of the present invention, there is provided an exposure apparatus, comprising: an illumination optical system for illuminating an original with ultraviolet light; a projection optical system for projecting a pattern of the original onto a substrate to be exposed; gas purging means for replacing an inside space, where optical components of at least one of said illumination optical system and said projection optical system are placed, with a particular gas; and passage means for mutually communicating spaces separated by said optical components, for gas purging.
In an illumination optical system or a projection optical system of a reduction projection exposure apparatus, an anti-reflection film (optical thin film) is formed on the surface of an optical element such as a transparent plate, a lens or a prism, for example. The provision of such an anti-reflection film is to efficiently direct light from a light source to a photosensitive substrate and also to prevent impingement of a flare or ghost on the photosensitive substrate. However, due to the property, such anti-reflection film may easily absorb the water content of gas. With an anti-reflection film having a water content adhered thereto, the spectral reflection characteristic on the surface thereof or the absorption coefficient on the surface may change by a small amount, causing a change in the spectral transmission factor. Usually, an illumination optical system and a projection optical system have optical elements with surfaces of a number of several tens in total. Thus, even if the change in spectral transmission factor per one surface is small, there may be a large change of spectral transmission factor in the whole optical system.
In accordance with the first aspect of the present invention, the ambience of optical components of the illumination optical system or the projection optical system may be replaced by a gas having substantially no water content, by gas purging. This is effective to prevent a change in transmission factor of the whole optical system (i.e., a change in illuminance or illuminance non-uniformness) resulting from a change in spectral transmission factor due to adhesion of water content to optical elements such as described above. Thus, a substrate can be exposed constantly with a correct exposure amount.
Absorption of exposure light or generation of xe2x80x9cblurxe2x80x9d material in a case where oxygen or ozone is present along the light path may be solved by accommodating optical components such as lenses in a casting of a sealingly closed structure and by filling the casing inside with an inert gas to thereby keep a low oxygen density. However, in a barrel where lenses are placed, there are spaces which are separated by lenses and the barrel structure. Proper gas replacement cannot always be done within these spaces. Namely, it is not easy to effectively prevent absorption of exposure light or generation of xe2x80x9cblurxe2x80x9d material.
In accordance with the second aspect of the present invention, the ambience of optical components of the illumination optical system or the projection optical system may be replaced by an inert gas, by gas purging. Additionally, there may be passage means communicated with the casing inside or the barrel inside, by which a low oxygen density can be kept within the space where glass materials are placed. This effectively prevents an oxidization reaction during an opto-chemical reaction process, and avoids generation of a resultant product (blurring material). Additionally, it effectively prevents or reduces ozone generation due to reaction oxygen by exposure light.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.